This invention relates generally to circuit breakers and, more particularly, to a trip assembly for use in a circuit breaker.
Circuit breakers typically provide automatic current interruption to a monitored circuit when undersired overcurrent conditions occur. These overcurrent conditions include, for example, overloads, ground faults, and short-circuits. An overcurrent is usually detected when the fault current generates sufficient heat in a strip composed of a resistive element or bimetal to cause the strip to deflect. The deflection triggers a trip assembly that includes a spring-biased latch mechanism to force a movable contact attached to a movable blade away from a stationary contact, thereby breaking the circuit. The strip is typically coupled to a heater which conducts the current-generated heat to the strip in a known manner. The current (within a predetermined threshold) at which the trip assembly is just prevented from acting yields the current rating for the circuit breaker. When the circuit is exposed to a current above that level for a predetermined period of time, the trip assembly activates and tripping occurs thereby opening the circuit.
To realize different current ratings, the compositions of the strips and/or heaters are varied. Varying the composition of a strip/heater causes the thermal behavior of each to change with a change in current rating. As a result, for each circuit breaker having a given current rating, a thermal analysis of the heater/strip assembly must be performed to ascertain the deflection of the strip versus time in response to heat generated by the heater.
For example, to produce an 80-amp circuit breaker and a 90-amp circuit breaker, the bimetal composition of the strips used in each breaker may be varied Each strip deflects differently in response to the same amount of heat. Generating the different deflection curves of each strip in response to a range of current is time intensive and is prone to error. Alternately, or additionally, the composition of the heaters used in each of the circuit breakers may be varied so as to alter the electrical resistance posed by the heater to through-going current. However, in this case, the varied compositions of the heaters produces different watts losses for each. This means that each heater generates a different amount of heat over a range of current. However, in order for the trip assembly for different breaker ratings to respond at a given overload, for example, 135% of the handle rating, in the same amount of time, the bimetal must deflect by the same amount. Thus, the heat generated in each case must be the same.
Accordingly, the strips attached to each heater, even if composed of the same bimetal, will deflect differently from one another. Thus, even if the heater composition is varied to achieve a desired current rating, multiple deflection curves must be generated.
Another disadvantage to the above approach is that for each additional current rating, at least one new material is introduced into the assembly process. Thus, for example, to manufacture a family of ten circuit breakers each having a unique current rating, as few as eleven and as many as twenty different materials must be kept on hand to assemble all ten circuit breakers. The multitude of different materials increases time, material and labor costs, and manufacturing complexity. For example, there may be a greater demand for a particular current rating, and to meet this greater demand, more materials destined for circuit breakers at the particular current rating must be kept on hand. Reducing the number of materials to achieve the same number of current ratings thus advantageously reduces the costs and complexities associated with producing circuit breakers having different ratings. The present invention exploits these and other advantages.
In an embodiment, a trip assembly for use in a circuit breaker includes a strip coupled to one of a first heater composed of a predetermined material and having a first cross-sectional area and a second heater also composed of the predetermined material and having a second cross-sectional area. The differences in the cross-sectional areas causes each heater to present a different electrical resistance to current passed through each heater.
In another embodiment, an arrangement of at least two heaters for use in circuit breakers having different current ratings includes a first heater composed of a predetermined material and a second heater also composed of the predetermined material. The second heater has a reduced shape relative to the shape of the first heater. As a result, the second heater presents a higher electrical resistance to through-going current than is presented by the first heater. The shape of the second heater may be reduced by varying surface area, thickness, or a cross-sectional area of the second heater.
In accordance with a method of assembling a trip assembly for use in one of a plurality of circuit breakers, the method includes forming a first heater composed of a predetermined material, forming a second heater also composed of the predetermined material such that the shape of the second heater differs from the shape of the first heater by at least one geometric parameter, and selecting and electrically coupling the first heater or the second heater to a thermally deflectable strip so as to achieve a desired thermal characteristic for a circuit breaker having a given current rating. The geometric parameter may be a surface area, cross-sectional area or a thickness.
The above summary of the present invention is not intended to represent each embodiment, or every aspect, of the present invention. This is the purpose of the figures and the detailed description which follow.